Changes in sea level are driven by a range of natural and anthropogenic forcings. To better understand the response of global mean thermosteric sea level change to these forcings, we compare three observational data sets to experiments of 28 climate models with up to five different forcing scenarios for 1957–2005. We use the preindustrial control runs to determine the internal climate variability. Our analysis shows that anthropogenic greenhouse gas and aerosol forcing are required to explain the magnitude of the observed changes, while natural forcing drives most of the externally forced variability. The experiments that include anthropogenic and natural forcings capture the observed increased trend toward the end of the twentieth century best.